rio of renji/kira/hinamori, i will just concentrate on renji since i dont know much information on the other two characters..

In eps 61 Aizen was explaining how his afraid of renji/kira/hinamori, what does he mean by being afraid of them?? they would disrupt/troublesome to his plans or afraid of their potential?? First time he saw them, he saw their work ethic and wanted to use them so he recruited them and split them up into other divisions. Lets have a breakdown:

Renji-
AIZEN claims that renji is the most troublesome/dangerous to him when they fought on the peak, he knows that renji wouldnt be useful to him so he doesnt recruit him into his or gins division. It could also be that of renjis personality that is hard to persuade or is he too smart to be outwit. So he breaks the trio up and sends them to other divisions. Then didnt AIZEN totally screwed up his own plans by sending Renji to Kuchikis division? You dont go and send a guy who is ambitious and a hard work ethic to a division that has a strong captain. And we all know how renji panned out, another good example is ikkaku how he panned layter on in the manga.

The serious question here is, why didnt aizen send renji to tousen's division, who is probaly one of the weaker captains....or maybe at that time tetsuo wasnt recruited to his alliance yet, or to any other inferior divisions.

I thought Renji was first sent to Zaraki's squad where he met Ikkaku, then got promoted to vice-captain in Byakuya's squad. Anyway. I think Aizen purposely didnt put Renji in any of his allies squads because Renji isnt good at following orders, hes to smart and rebellious so Aizen thought that if he trusted Renji, hed screw up by following his heart. YAY RENJI. As for Hinamore and Kira, I think that he liked Hinamoris kido skills and Kira's zanpakutoh and thought theyd be useful to have on his side till Hinamori went obsessive and Kira turned into a coward. So basically, they were a threat until they all had emotional breakdowns.

Afraid? Hmmm Aizen being afraid of three simple rookies seems completely out of place, I don't remember him saying that. But he definitely wasn't afraid of their potential, rather, he saw the potential in them and wanted to use that potential, which is why he took them under his wing so he could manipulate them. This only worked with Hinamori and Kira though, as Renji was pretty much too rebellious and stubborn So Aizen just gave up on Renji and focused on HInamori and Kira. He succeeded in manipulating them by making them his and Gins Lieutenants. Aizen didn't send Renji to Tousen's division because he basically knew he wouldn't be able to manipulate Renji, he was too troublesome so he just completely gave up on him. And yep Tousen was with Aizen from the very beginning too, so he did have that option but there was no point. In addition, Renji was sent to the 11th division initially as has been said, there he met Ikkaku and trained with him so he could one day take over Byakuya.

Hehe, this reminded me of how intricate all the background infromation is It all blends together pretty well.

Aizen has nothing to be afraid off, Renji was simply being used. Renji likes to act on his own sometimes, whether the captain tells him something or not. That was troublesome to Aizen, so he sent Renji away.

Aizen's a perfectionist, in a sense. He's not really afraid of people's strength, just really abhors people's willpower. Renji, Kira and Hinamori have too much of that. How you ask? Renji's just being what he always has been: stubborn, loyal to friends and etc. As for Hinamori, AIzen probably foresaw already how attached the girl would be to him. This why he tried to get Kira to kill her because he didn't want to dirty his own hands. And as for Kira, well...i don't really know. There's just something about him that you feel he's gonna find out what you're thinking if he stares at you too long.